Publications and other reference materials referred to herein, including references cited therein, are incorporated herein by reference in their entirety and are numerically referenced in the following text and respectively grouped in the appended Bibliography, which immediately precedes the claims.
There are many applications that would benefit from a noninvasive optical means of viewing objects embedded within turbid media or otherwise obstructed by other objects. In military and security applications the ability to see through obstructions or turbidity is of great importance for example for detecting the presence of concealed weapons under clothing or viewing enemy forces concealed by fog or cloud cover.
In medical diagnostics the most common methods for diagnosing abnormalities of internal organs are based on circumstantial evidence. For example, high cholesterol levels in the blood might be circumstantial evidence for vascular diseases. Obviously, the physician can use endoscopes to obtain a much better perspective of the extent of the disease. The ability to “see” the organ, i.e., to measure its optical properties, results in a wealth of clinical information. The reason for this is that most of the biological molecules absorb light in the optical regime of the spectrum. Therefore, it is relatively simple to distinguish between different biological tissues with optical devices. However, endoscopy and similar procedures are invasive methods and therefore cannot be used in many cases or as frequently as the physician would like.
For these reasons, noninvasive methods were developed to diagnose internal organs. However there exist few, if any reliable noninvasive techniques to measure the optical properties of these internal organs. As a consequence, most of the ubiquitous methods rely on circumstantial evidence for diagnosing the internal abnormalities. For example, in x-ray mammography most of the tumor findings are due to the presence of micro-calcifications, which gives only circumstantial evidence of the tumor's presence.
The inability to see through biological tissue is only a specific example of a broad family of related problems pointing to the apparent futility of using optical radiation for direct imaging through a turbid or obstructing medium. This problem is ubiquitous in everyday life. Fog, clouds, turbid water, and milk are just a few examples from everyday encounters with such optically turbid media. In particular, biological media are highly diffusive for visible and infrared light. As a result, an optical image is severely deteriorated, even after passing through a few millimeters of tissue.
Despite this severe shortcoming, it is clear that a reliable non-invasive optical technique would be a very useful clinical tool in diagnosing in-vivo internal organs. Moreover, the use of light as a diagnostic tool has many advantages over other non-invasive diagnostic techniques. Unlike x-rays, light is not oncogenic, it has the potential of giving a much higher resolution than ultrasound imaging, and the required equipment is potentially much cheaper than MRI. These benefits and technological progress in optical equipment have encouraged researchers to develop optical diagnostic tools for turbid biological systems.
Although many methods based on coherence, polarization, diffusion properties etc. have been developed [1], two techniques appear to be the most promising: the time-domain and frequency-domain techniques. [Other techniques, such as optical coherence tomography, which utilizes short-coherence-length light, can be regarded as equivalent to the short pulse illumination method.]
The most direct and one of the most popular approaches to seeing through a turbid medium is the first-light or time-domain technique. In this method the first arriving light, which contains the undistorted image, is separated from the scattered light by a very fast time-gating camera [2-6]. In ref. 3, the authors suggest that a resolution of a few millimeters is achievable in a system with a temporal resolution of about 10 ps. This method demands complex and very expensive equipment.
While the time-domain technique gives superfluous information, and the requirement of the measurement equipment is to distinguish between the informative (i.e., ballistic) portion of the signal and the noise (i.e. diffusive portion), the frequency-domain techniques usually results in less information. In the frequency-domain approach [1, 7-9] the laser light is RF modulated (MHz-GHz), and the measurement of the amplitude and phase of the modulated diffusive light take place at the boundaries of the medium. Then, a numerical reconstruction takes place in which the light distribution in the medium is calculated from the collected data.
The frequency-domain technique is considered to be simpler, more reliable and more economic than the time-domain method, since it needs much simpler equipment. Moreover, unlike the time-domain method, the detected signal in this technique can be considerably stronger since it includes the entire signal energy, not merely the ballistic or quasi-ballistic portion, which can be quite small in a highly turbid medium,
In principle, the two methods are equivalent since the latter one can be regarded as the Fourier counterpart of the former. However, except for small technical details, the main difference between the two is the spectral range. In the time-domain techniques (for medical purposes) pulses of about a few picoseconds are used, which is equivalent to a spectral range of more than 100 GHz. However it is difficult to modulate light at frequencies greater than 10 GHz. Therefore the concept is different: instead of collecting the data in time, in the frequency-domain method the data is collected in space (on the medium's boundaries). Such a method requires complicated numerical algorithms, is sensitive to boundary conditions, and its spatial resolution is limited by the medium's diffusion length.
It is therefore a purpose of the present invention to provide a method for determining the optical temporal response of a medium to a short optical pulse excitation, at least a portion of which is indicative of the position and shape of an object or objects embedded in an optically turbid medium and/or obstructed by other objects that are at least partially transparent.
It is another purpose of the present invention to provide a method for determining the optical temporal response of a medium to a short optical pulse excitation, at least a portion of which is indicative of the position and shape of an object or objects embedded in an optically turbid medium and/or obstructed by other objects, that can be carried out without using short pulses and fast detectors.
It is a further purpose of the present invention to provide a method for determining the optical temporal response of a medium to a short optical pulse excitation, at least a portion of which is indicative of the position and shape of an object or objects embedded in an optically turbid medium and/or obstructed by other objects, that can be carried out with relatively inexpensive equipment.
Further purposes and advantages of this invention will appear as the description proceeds.